The present invention relates to a skid control system which includes a skid detector which is responsive to the deceleration of the vehicle wheel and which, in one embodiment, provides an output signal after a predetermined amount of slip of the wheel is incurred, and in another embodiment, provides an output signal when a predetermined wheel deceleration is attained. In the one embodiment, the skid detector includes means for delaying the response of wheel deceleration detector until the wheel has continued to decelerate above the threshold for a predetermined period. The skid detector is preferably used in combination with a holding circuit which, in selected instances, maintains the brakes relieved after the dissipation of the output of the skid detecting circuit for a predetermined period and a skid recovery detector which overrides the holding circuit. According to the present invention, this holding circuit advantageously includes a circuit for providing either one or two preestablished rates of charge of the capacitor and two preestablished rates of discharge of the capacitor to modify the performance of the skid control system in accordance with such factors as the elapsed time that the wheel is in a skid condition as indicated by the skid detector and the existence of a signal from the skid recovery detector.
In a first embodiment of an exemplary skid control system according to the present invention, the holding circuit capacitor is charged at one preestablished rate in response to an output from the skid detector. An output signal from the skid recovery detector, however, overrides the skid detector output to terminate the charging of the hold-on circuit capacitor. Any time the hold-on capacitor is not being charged, i.e. there is no skid detector output signal, or if there is a skid detector output signal but it is overridden by a skid recovery detector output signal, the hold-on circuit is discharged at a preestablished low discharge rate. The hold-on capacitor is discharged at a high rate when there is no skid detector output signal and the charge on the hold-on capacitor is below the predetermined level which maintains the brakes in the released condition. The hold-on capacitor is also discharged at the high rate in response to an output signal from the skid recovery detector when there is no skid detector output signal.
The first embodiment of the skid control system according to the present invention is also provided with a circuit for limiting the saturation of the skid detector to adjust the duration of the skid detector output signal in accordance with the level of wheel deceleration, i.e. the skid detector output signal duration is increased for wheel decelerations above a preselected deceleration level. The saturation limiting circuit is rendered inoperative to increase the duration of the skid detector output signal either during sufficiently low wheel speeds or during the period that the hold-on capacitor charge level is above the pre-established level.
In a second embodiment of a skid control system according to the present invention, the hold-on capacitor is charged at a low rate when the wheel exceeds a low deceleration threshold and is charged at a high rate when the wheel exceeds a low deceleration threshold and the wheel speed is below a predetermined low speed. The hold-on capacitor is generally discharged at a rate which is in accordance with the level of wheel acceleration. When the wheel exceeds a high level of wheel acceleration the hold-on capacitor discharge rate is substantially increased. The charge level on the hold-on capacitor is compared with two reference charge levels. A threshold representative of a lower charge level is in effect when the wheel exceeds the low deceleration threshold or the wheel speed is below a predetermined low speed. A threshold representative of a higher charge level is in effect at other times.
Further features and provisions of this invention will be apparent in view of the detailed description of the preferred embodiments set forth hereinafter.